How do I use a laser distance measure accurately?

A laser measure is the fastest way to size a room and the only practical way to measure a ceiling height on your own. It is also capable of being confidently and repeatably wrong, and almost all of that comes down to two things: which end of the tool it is measuring from, and whether the beam is actually landing where you think it is.

Safety

What you'll need

• Laser distance measure
• A stiff white card as a target (optional)
• A tape measure to cross-check

Step by step

1. Set the reference point before anything else.Every unit measures from a selectable datum: the back of the case, the front edge, or a fold-out foot for measuring out of a corner. Get this wrong and every reading is out by the length of the tool — typically 100 to 130mm. It is the single biggest source of error and it is invisible in the number on the screen.
2. Use the fold-out foot for internal measurements.Flip out the rear foot, hook it into the corner, and the tool automatically switches its datum to the pivot point. This is what makes a laser genuinely good at alcoves, reveals and room widths, where a tape has to be bent into the corner.
3. Hold it square to the target.A laser measures the straight-line distance to whatever the beam hits. Held at an angle, it reads the hypotenuse, which is always longer than the perpendicular distance you wanted. Rest the base flat against the wall you are measuring from.
4. Check where the dot has actually landed.Over a long distance in daylight the dot is faint and easy to lose. If you cannot see the dot, you do not know what you measured — it may have gone through a doorway and hit a wall in the next room, which reads as a plausible number.
5. Use a target card on dark, shiny or transparent surfaces.Dark paint, gloss, glass, water and polished stone either absorb the beam or reflect it away. Hold a piece of white card at the target point and measure to that. Glass in particular gives a reading to whatever is behind it.
6. Take three readings and expect them to agree.A good unit repeats to a millimetre or two. If three readings of the same distance disagree by more than that, the beam is landing somewhere different each time and none of them can be trusted.
7. Cross-check against a tape once per job.Measure one dimension both ways at the start. It catches a wrong reference setting, a unit left in feet and inches, and a flat battery producing lazy readings, before any of them cost you material.
8. Use a tape for anything short.Below roughly half a metre most units are outside their minimum range or their accuracy is a large fraction of the measurement. For joinery-scale work a tape or a rule is quicker and better.
9. Use the memory and the running functions rather than a notepad.Most units add, subtract, and calculate area and volume from stored readings. Adding four wall lengths in the tool avoids the transcription errors that happen between a screen and a piece of paper.

Tips

• The indirect or Pythagoras function measures a height you cannot reach by taking two readings from one position — useful for a gable, a stairwell or a ceiling over a stairwell.
• Cold kills the batteries and the display, so a unit left in a van in winter will behave oddly for the first few minutes indoors.
• For measuring up a whole house, a laser is transformative because you can do it alone. That is the real reason to own one, rather than accuracy.

Common mistakes

• Leaving the reference point on the wrong setting — Every reading is then out by the length of the tool, consistently and invisibly. Because all the numbers are wrong by the same amount, nothing looks suspicious until something is cut.
• Measuring through a doorway or an open serving hatch — The beam does not stop at the plane of the wall. It carries on and reports the distance to whatever it finds, which is a perfectly believable number for the wrong room.
• Trusting a reading you did not see the dot for — Without seeing where the dot landed you have measured to an unknown point. In bright daylight this is the normal condition, not an unusual one.

If it doesn't work

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